Tuesday, June 2, 2009

Histograms and Star-Count Estimate


To examine the distributions objects as galaxies or stars more clearly, Beth suggested plotting a histogram. This lead to a lot of Idl instruction reading, and a moderate amount of her help. But by the end of the day I had a good plot of the distribution of objects, clearly (thank goodness) showing tall spikes at the galaxy end of the spectrum, and a moderate spike at the star end.
Next I set out to make a hist. of the range of FWHM amongst the stars. I did so by learning how to use a "where statement" (Beth's favorite!), to pick out just those objects in the sample whose classifications had been assigned a value between 0.7 and 1.0. Debugging was still needed.


Finished up (learning how to) making the FWHM histogram.
Also read through the paper Stars in the Hubble Ultra Deep Field to learn how they seperated the galaxies from the stars in thier catalog of apporx. 10,000 objects. Though they used some tricky super-mathematical techniques, it appears to have boiled down to their light distribution, and then a little magnitude profiling. This gave them 46 unresolved objects (non-galaxies, at least until further narrowing down). Of that set, they used what spectroscopy that they could, and elimination of dimmer than an i magnitude of 27, to result in 26 stars.

Given the number of objects, we had a bit of a scare, anticipating that if proportional, our catalogs would then result in a mere 100 or so stars. But never fear! Arithmatic is here!
The area of the HUDF is 11 sq. arcmin, meaning there were 2.364 stars per sq. arcmin. Our GOODS survey data (N and S fields combined) constitute 320 sq. arcmin, resulting in 756.48 stars.

I'm glad- an estimated 750 stars is probably a lot more useful. Or maybe not? Guess we'll have to see how the numbers boil down.

For Tomorrow:

I began writing code to make some color-magnitude diagrams today, so I'll hopefully finish them up by our group meeting in the afternoon.

1 comment:

  1. I just realized our folly in our original estimate of 100 stars... we considered the number of sources in the entire HUDF catalog, which I think is all the way down to 29.5 mag! So at the magnitude limit of GOODS there would be far fewer galaxies. Whew. I'm eager to see how many stars you get by a simple cut on CLASS_STAR and I'm eager to look at the CMDs of stars (and of the galaxies too). CMDs in V, V-I will be the first ones to look at (b/c I'm the most familiar with what CMDs look like in those filters).